Programming Constructs

Basic Input and Output


The INPUT directive is used to enter data interactively while the program is running. It issues prompts to the user and processes the responses. PRINT statements may be used to format and output printable data to a monitor or printer, as well as into a file. PxPlus I/O statements need to reference a channel number in order to access a device or file. The relationship between a channel and the physical file or device must first be established via the OPEN directive.

Other I/O directives (READ, WRITE, etc.) are used specifically for transferring data in and out of files. See File Processing Directives.

For information on I/O processing in a GUI environment, see Graphical User Interfaces.

Opening/Closing Devices and Files

Most input/output operations in PxPlus require a channel number (chan) to identify the connection to a specific device, interface or file. These are established using the OPEN directive, which associates a logical number (normally between 0 and 127) with a physical file or device. Once the chan is defined, the program will be able to access the input or generate output simply by referencing that number.

In theory, all I/O statements use channel numbers; however, the console (keyboard and display screen) is defined as channel 0 by default and may be omitted from INPUT or PRINT statements intended for immediate display.

For files, the OPEN process also sets a pointer to the beginning of the opened file and allocates system resources. The actual number of files that can be opened at any one time depends on the operating system. See File Handling for information on using OPEN for file I/O operations.

The basic syntax for an OPEN statement is:

OPEN (chan,fileopt ]) string$



Logical channel number to be assigned.


Various options used for controlling the contents and characteristics of a data file. See Processing Data Files.


Name of the file or device to open. The string expression can include a specialty file name or file tag (e.g. *MEMORY*, [RPC], etc.).

Several OPEN keywords are also available for specific types of file access operations, including INPUT, LOCK, PURGE and LOAD.

Most I/O operations begin with an OPEN directive to establish the relationship with the target device or file.


PRINT "Today's date is ",DAY

OPEN (1)"*WINPRT*;HP Laser Jet;Orientation=Landscape"

PRINT (1)@(0),"Customer",@(45)," Balance"

In this example, the first statement simply outputs text to the screen, which is the default syntax for PRINT. The second PRINT statement outputs text to channel (1), which is the identity of a printer made available for use via the preceding OPEN directive.

The CLOSE directive closes the connection to one or more files/devices and allows their channel number(s) to be reused. The basic syntax is:

CLOSE {(*) | (chan) [, (chan) ...]}



An asterisk denotes "all OPEN local channels" except channel 0.


Channel number.

Local files are closed automatically on a BEGIN or END statement. All files are closed at the end of a user session or whenever a START directive is issued.

See Also

Input Statements
Output Statements